Ever since 1477, when the Austrian Archduke Maximilian gave his fiancée, Mary of Burgundy, a diamond ring as he proposed to her, a diamond has been the time-honoured way to declare everlasting love and seal an intention to wed. The custom of wearing the ring on the third finger of the left hand, which was thought to contain a vein that led straight to the heart, has been around since the ancient Egyptians. But pity the poor male suitor today, who is probably still paying off his university loan, living with his parents to save up for a mortgage and very possibly earning less than his bright and beautiful love.

In Diamond Street: The Hidden World of Hatton Garden (Hamish Hamilton, £20), Rachel Lichtenstein looks perceptively at an area of London that for the last 200 years has been at the heart of the diamond and jewellery business. Third- and fourth-generation diamond merchants discuss how times have changed. One of them relates how even though nowadays couples live together for years, and despite the temptations of online shopping, they are drawn to Hatton Garden the moment they have agreed, in theory at least, to spend the rest of their lives together. It can often be a fraught trip. The young man usually tries to guide his future wife to the less expensive stones, while she usually falls for something well beyond his reach.

It is understandable. Ever since De Beers launched its brilliant "Diamonds are Forever" marketing campaign in 1938, the size of the diamond has often been confused with the magnitude of the man's love. Agreeing on the ring is perhaps the first real test of compromises a couple will have to make during their life together. Both the commitment and the ring can become expensive mistakes. One merchant recalls that some couples are unable to agree on a purchase and leave looking disgruntled. Mostly, however, they pick a stone they feel symbolises their love and walk off into the sunset with glowing cheeks and hands clasped.

An increasing number of men select a ring before they propose. Despite the risk that they might be turned down or wrongly interpret their loved one's taste, at least they can buy something within their price range without feeling guilty. If they get it wrong, it takes a brave woman to complain, especially if her biological clock has started to tick loudly. Many young men resist commitment; so if pretending he has chosen the most beautiful ring ever seen gets them to the altar, it's probably well worth a little white lie.

The man not getting it quite right could also be why an increasingly number of women are buying their own diamonds — although significantly they choose earrings and bracelets rather than a ring, as a symbol of their independence. A diamond's position as a girl's best friend could be extremely useful if things go wrong.